Multisensor with directly coupled rotors

ABSTRACT

A multisensor includes direct rotor coupling. A pair of rotors is mechanically coupled by a torsion bar whose stiffness largely determines the resonant frequency of the counteroscillating rotor pair. The axis of the torsion bar is coincident with the common axis of rotation of the rotors. Piezo bimorphs are fixed to opposed surfaces of radial vanes of the rotors which suspend the torsionally-coupled rotor pair with the case, isolating it from external angular vibration that could interfere with the resonant frequency of the rotor pair subassembly. The bimorphs receive signals for driving the oscillations of the rotors. Taken together, the stiffnesses of the vanes and the torsion bar substantially determine the common resonant frequency of the counteroscillating pair of rotors.

CLAIM TO PRIORITY UNDER 35 U.S.C. 120

This application is a continuation of U.S. patent application Ser. No.08/904,927 of Robert E. Stewart titled "Multisensor with DirectlyCoupled Rotors" filed Aug. 1, 1997 now U.S. Pat. No. 5,932,805.

BACKGROUND

1. Field of the Invention

The present invention relates to economical inertial navigation units(IMU's) for short range, relatively low accuracy guidance applicationssuch as munitions. More particularly, this invention pertains to amultisensor of the type that employs paired triads of accelerometersmounted upon counteroscillating platforms for directly measuring linearaccelerations and for determining rotation rates from Coriolis forceswith respect to a three-axis system.

2. Description of the Prior Art

IMU's measure space-dependent rotations and accelerations with respectto orthogonal space axes. Their designs are beset by numerousdifficulties as this requires the simultaneous measurement of sixindependent variables. For example, gyroscopes of the ring laser andfiber optic type require a lasing cavity dedicated to each input axis,mandating a total of three lasing cavities, an expensive undertaking,for obtaining three of the six measurements required of an IMU. (Anexample of a laser device for measuring rotation about three axes isshown in United States patent, property of the assignee herein, U.S.Pat. No. 4,795,258 of Martin entitled "Nonplanar Three-Axis Ring LaserGyro With Shared Mirror Faces".) IMU's employing spinning wheel gyrosmust deal with such gyros' limitation to measurement of rotation withrespect to two axes are limited to two axes of measurement,necessitating the use of an additional drive mechanism for the thirdinput axis. Again, this does not in any way account for the additionalcomplication introduced by the remaining measurements of accelerationswith respect to the axes.

Simplicity and economy are particularly significant in the design ofIMU's for munitions guidance and like applications. Such uses arecharacterized by non-reusable payloads, limited flight durations andonly moderate accuracy requirements. One economical type of system formeasuring both rotation rates and linear accelerations with reference toa set of three orthogonal axes is a multisensor mechanism taught in aseries of United States patents, also the property of the assigneeherein (U.S. Pat. No. 4,996,877 entitled "Three Axis InertialMeasurement Unit With Counterbalanced Mechanical Oscillator"; U.S. Pat.No. 5,007,289 entitled "Three Axis Inertial Measurement Unit WithCounterbalanced, Low Inertia Mechanical Oscillator"; and U.S. Pat. No.5,065,627 entitled "Three Axis Inertial Measurement Unit WithCounterbalanced, Low Inertia Mechanical Oscillator.") The devicesdisclosed in the referenced patents employ piezoelectric drivemechanisms for driving a pair of counterbalanced platforms to oscillateout-of-phase about a common axis within a housing or case.Accelerometers housed in a vacuum to avoid the effects of gas dampingare mounted in a tilted manner (for measuring variables in orthogonalplanes) to radially-directed elements of the platforms provide measuresof both linear acceleration and rotation. The latter (rotation) valuesare derived from the (Coriolis) forces sensed by the accelerometers atthe resonant frequency of the counteroscillating structure.

The oscillatory motions of the rotors of the multisensors taught by theabove-identified patents are coupled to one another through the casethat houses the mechanism. Each rotor comprises three radially-directedrotor arms. An accelerometer is fixed to each rotor arm. The rotor armsalternate with rotor platforms, each including three radially-directedwebs. Piezoelectric elements are mounted to either side of the outerwebs. The elements are appropriately-poled so that an input drivevoltage signal simultaneously induces compression and tension at theopposite surfaces to cause predetermined bending of the webs thatresults in oscillation of the rotors. The central web is relativelystiff. Such stiffness is the major factor that determines the natural orresonant frequency of the rotor.

Each rotor is bolted only to the case for support. As a result, the caseprovides the only path for transferring energy between the oscillatingrotors. As mentioned earlier, measurement of rotation rate throughsensing of Coriolis acceleration relies upon the demodulation of anoutput signal whose frequency is equal to the resonant frequency of thepaired rotors. A single resonant frequency is assumed. Theabove-described design is subject to factors that can complicate themeasurement of rotation rate to a significant extent. Many of suchcomplications follow from the only-indirect coupling of energy (i.e.through the case) between the paired rotors.

Numerous arrangements can act to weaken the already-indirect coupling ofenergy. For example, many multisensor applications require that the casebe hard-mounted to a body. In such applications, the mechanicalimpedance of the outside world is integrated into the coupling of therotors so that the transfer of energy between the oscillating rotors issubject to attenuation in complex, and sometimes-unforeseen, ways. Thus,the accuracy of rotation rate measurement can vary as a function ofapplication and changes in mechanical impedance.

Solutions to problems relating to weakly-coupled rotors to overcome theleakage of energy and problems related to differential rotor frequenciesare quite complex and often expensive to implement. One solution,adjusting the relative amplitudes of the rotor drive voltages, canintroduce bias effects, complicate system electronics, etc. Anothersolution is to mount the multisensor case on isolators so that thedevice is no longer hard-mounted to the outside world. While essentiallysolving the problems of external impedances, isolation-mountingmultiplies mechanical complexity, size and cost, often to a significantextent.

SUMMARY OF THE INVENTION

The preceding and other shortcomings of the prior art are addressed bythe present invention that provides a multisensor that includes a pairof rotors, each of which includes a plurality of radially-directed rotorarms. An accelerometer is fixed to each rotor arm. Each of the rotors isarranged to oscillate about a common axis. Means are associated witheach rotor for oscillating the rotors substantially 180 degreesout-of-phase with respect to one another as are means for directlycoupling the rotors to one another along the common axis. Such means hasan axis of symmetry that is coincident with the common axis and includes(i) a cylindrical central member of a first diameter and (ii)cylindrical end members of a second, larger diameter.

The foregoing and other features and advantages of this invention willbecome further apparent from the detailed description that follows. Thisdescription is accompanied by a set of drawing figures. Numerals of thedrawing figures, corresponding to those of the written description,point to the various features of the invention. Like numerals refer tolike features throughout both the written description and the drawingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a counterbalanced multisensorin accordance with the invention;

FIGS. 2(a) and 2(b) are schematic views of an accelerometer and of thecounteroscillatory structure, respectively, of a counterbalancedmultisensor;

FIG. 3 is a cross-sectional view in elevation of the multisensor of theinvention; and

FIG. 4 is a top plan view of a rotor in accordance with the inventionwith flexure vanes shown in shadow outline to illustrate the operationthereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an exploded perspective view of a counterbalanced multisensorin accordance with the invention. The principal components andarrangements of the invention may be seen to include a multisensor case10 that houses various multisensor mechanisms and provides a means formounting to a body. Active mechanisms of the multisensor include a pairof rotors 16, 18 joined at a common torsion flexure 20. It shall be seenbelow that the arrangement of the rotors and the torsion flexureprovides the multisensor with enhanced performance over prior artarrangements that rely solely upon a path through the case 10 forcoupling rotational energy between the rotors 16 and 18. In theinvention, the rotors 16, 18 will be seen to oscillate efficiently and180° out-of-phase with respect to one another at a single resonantfrequency ω_(D).

Hybrid substrates 22, 24 of circular shape are arrayed concentric withthe common axis of oscillation 26 of the rotors 16, 18. The substrates22, 24 include preamplifier electronics for use in servo-controlling theaccelerometers fixed to the rotor. Disk-like rotor support plats 28, 30,and electronics housings 31 (each having an associated end plate 32)complete the major mechanical structures of the multisensor.Feedthroughs 33 within the peripheries of the rotor support plats 28, 30provide communication with external electronics apparatus. Suchapparatus inputs and receives signals for use in the servo-control,drive and output measurement and processing functions associated withthe multisensor.

Both low frequency linear and resonant frequency accelerations aresensed by responsive mechanical apparatus. Accelerometer assemblies 34,36 and 38 are angularly inclined adjacent the distal ends of arms 40, 42and 44 respectively of the rotor 18. Like assemblies are fixed adjacentthe distal ends of arms 46, 48 and 50 of the rotor 16. Each of thelatter accelerometer assemblies is mounted at a complementary angle tothat of the corresponding assembly of the rotor 18.

Radially-directed vanes 52, 54 and 56 are interspersed with theradially-directed rotor arms 40, 42 and 44 of the rotor 18 while asimilar arrangement pertains to the relationship between flexure vanes58, 60 and 62 and the rotor arms 46, 48 and 50 of the rotor 16.

The oscillatory movements of the rotors 16 and 18 with respect to thecommon axis 26 result from forces applied to the vanes 52 through 62.Such forces are induced by the application of dither drive voltages tosets of vane-fixed piezoelectric bimorphs, such as the bimorphs 64, 66located at opposed surfaces of the vane 56. Pairs of driving bimorphsare fixed to two vanes of each rotor with a pair of pickoff bimorphsfixed to the third vane. The pairs of driving piezoelectric bimorphsfixed to the opposed surfaces of the rotor vanes alternately expand andcontract in response to applied dither drive voltages causing the vanesto impart angular motion to the rotors 16 and 18 and, of course, to theaccelerometer triads mounted thereto while voltages induced in theflexed third vane induce a pickoff signal for use in regulating theamplitude of oscillations. The desired 180° out-of-phase oscillatoryrelationship between the rotors 16 and 18 may be accomplished through anumber of arrangements including application of out-of-phase drivesignals to the bimorphs of corresponding vanes of the opposed rotors 16and 18 as well as affixation of oppositely-poled bimorphs to thesurfaces of corresponding radial vanes of the rotors 16 and 18 whileapplying identical drive voltages.

An enlarged view of the representative accelerometer 38 that is fixed tothe arm 44 of the rotor 18 is identified and located by means of anarrow 68. In FIG. 1 an orthogonal coordinate system is superimposed uponthe accelerometer 38 for indicating various axes whose significance willbecome further apparent below. An axis denoted OA is aligned within thehinge of the accelerometer 38. This axis is perpendicular to an axis PAthat passes through the center of gravity in the plane of the pendulousmass of the accelerometer 38. An input axis IA is perpendicular to theplane of the axes OA and PA.

The theory of operation of a counterbalanced multisensor will bediscussed with reference to FIGS. 2(a) and 2(b), each of which providesa schematic view in perspective of a central aspect of the system. InFIG. 2(a) there is illustrated a representative accelerometer with axesand vectors marked thereon indicative of a Coriolis acceleration output.FIG. 2(b) illustrates the operation of an array of accelerometers withinthe counterbalanced system of an accelerometer.

In such a system, six micromachined silicon accelerometers permitcomplete measurement of the linear acceleration and angular rate of ahost vehicle in body-fixed coordinates by making direct measurements oftotal acceleration with respect to the body axes. The total accelerationmeasurements are corrected for the Coriolis acceleration which is usedto determine body angular rate.

Referring specifically to FIG. 2(a), Coriolis acceleration is measuredas the cross product A_(C) =2Ω×V where Ω is the body angular rate vectorand V is the instantaneous relative velocity of the sensor mount withrespect to the host vehicle. Thus the sensor, when set in motion with arelative velocity in the direction of the output axis OA, permits anangular rate about the pendulous axis PA to be observed as part of anacceleration measured along the input axis IA. The Coriolis accelerationmeasurement permits angular rate Ω to be observed with accelerometers ofthe multisensor. However, a procedure is then required for separatingthe linear and Coriolis acceleration components from one another.

One approach to such separation is to induce a sinusoidal relativevelocity in the form

    V=V.sub.o Sin ωt                                     (1)

The corresponding Coriolis acceleration is then

    A.sub.c =2Ω×V.sub.0 Sin ωt               (2)

Thus, as long as an additive linear acceleration does not produce acomponent in the frequency band of the Coriolis part, then demodulationat ω_(D) of the accelerometer's output yields a measurement of angularrate about the PA axis of (each) accelerometer. Similarly, the lowfrequency linear acceleration component is obtained by low-passfiltering. As mentioned earlier, linear acceleration is near d.c. whilethe angular velocity is modulated with the much higher frequency ω_(D).

Referring now to FIG. 2(b), the accelerometers are oriented with theirinput axes inclined with respect to their corresponding planes of motionto move at 180° out of phase with one another on the counter-vibratingrotors 16, 18. In the presence of an angular rate Ω along PA and anacceleration component A along IA (refer to FIG. 2(a)), the totalacceleration measured by each is:

    A.sub.1 =A+2ΩV.sub.0                                 (3)

    A.sub.2 =A-2ΩV.sub.0                                 (4)

    Thus,

    A=1/2(A.sub.1 +A.sub.2)                                    (5)

    and

    Ω=(1/4V.sub.0)(A.sub.1 -A.sub.2)                     (6)

A more accurate procedure for in defining Ω(t) is derived from thegeneral relations

    A.sub.1 (t)=A(t)+2Ω(t)V.sub.0 Sin ωt+e.sub.1 (t)(7)

    A.sub.2 (t)=A(t)-2Ω(t)V.sub.o Sin ωt+e.sub.2 (t)(8)

where e₁ (t) and e₂ (t) are high-frequency vibration effects that may beconsidered as error terms. Upon elimination of A(t), the followingresults

    Ω(t) Sin ωt=1/4V.sub.o [A.sub.1 (t)-A.sub.2 (t)]-(1/4V.sub.0)[e.sub.1 (t)-e.sub.2 (t)]                (9)

Assuming that none of the structural natural frequencies of themultisensor are close to the modulated band of Ω(t), then e₁ (t) and e₂(t) basically cancel one another, leaving the following expression forΩ(t):

    Ω(t)=(1/4V.sub.0)[A.sub.1 (t)-A.sub.2 (t)]           (10)

FIG. 3 is a cross-sectional view in elevation of the multisensor of theinvention taken. It is to be understood that the sectional view is ofthe device in assembled form. As shown, the torsion flexure 20 is fixedby suitable adhesive such as the resinous catalytic adhesive marketedunder the trademark "EPOXY" (alternative: laser welding), at itsopposed, enlarged-diameter ends 69, 70 to aligned apertures within thecentral hubs 71, 72 of the rotors 16 and 18 respectively. In an actualembodiment of the invention, the torsion flexure 20 was of overalllength 0.34 inches with an elongated central portion 0.197 inch long andof 0.074±0.001 inch diameter. Enlarged-diameter end sections 69, 70 weremachined to 0.1960±0.0005 inches. The torsion flexure 20 wasmanufactured of 300 Series corrosion-resistant stainless steel ("CRES").When assembled, a clearance d of 0.02 inches was provided between theadjacent surfaces of the rotors 16, 18. The material composition of thetorsion flexure 20 is chosen for compatibility, in terms ofthermally-induced axial expansion, with the material of the case 10. Thecase 10 may, in turn, be fabricated of such materials as Hi-Mu 80 (anickel-iron alloy) or Carpenter 49.

As can be seen from FIG. 3, the substrates 22, 24 are affixed to theoutwardly-facing surfaces of the rotors 16 and 18 respectively. Therotor support plats 28, 30 are laser welded to opposed ends of theinterior cylindrical cavity of the multisensor case 10. The laser weldprovides the necessary hermetic seal for maintaining a vacuum inside thecase 10.

While the substrates 22, 24 are fastened to the arms of the rotors 16,18, the mounts are fastened to the plats 28, 30 by means of bolts,screws or the like. As mentioned earlier, the support plats 28, 30 arefixed to the ends of the case 10, rendering the rotors 16, 18 partiallycase-fixed. However, the direct coupling provided by the torsion flexure20 effectively channels the major portion of the torsional energy thatis alternately stored as potential energy and expended as kinetic energywithin the subassembly comprising the rotors 16, 18 and the torsionflexure 20. Thus, unlike the prior art, which relies entirely upon themultisensor casing as a medium of transfer of energy between the rotors,only a small amount of such energy is transferred through it in thepresent invention.

FIG. 4 is a top plan view of a rotor 74 in accordance with the inventionwith flexure vanes shown in shadow outline to illustrate the operationthereof. As can be seen, an enlarged-diameter end 76 of a dumbell-shapedtorsion flexure is either adhesively fixed or laser-welded within acentral aperture 78 of the hub 80 of the rotor arm. As mentionedearlier, the center of the aperture 78 is aligned with the shared axisof oscillation of the rotor pair.

As also mentioned earlier, the rotor 74 comprises two distinctradially-directed structures that emanate from the central hub 80. Theseinclude two overlapping, Y-shaped assemblies that are symmetrical withrespect to one another. Three equiangularly-spaced rotor arms 82, 84, 86alternate with piezoelectric-fixed assemblies 88, 90, and 92. Asmentioned earlier, accelerometers are fixed to the rotor arms 82, 84, 86for measuring both linear and Coriolis accelerations.

Referring to the representative drive assembly 88, this is seen toinclude a flexible vane 94 that is fixed to, and radiates from, thecentral hub 80 which is, in turn, fixed to the end 76 of the torsionflexure. This is in contrast to drive assemblies of prior artmultisensors, such as taught by the above-referenced United Statespatents, in which each assembly includes three radial elements--a stiffcentral member and a lateral pair of thin vanes or webs. Pairs of poledbimorphs are fixed to the lateral webs for driving each assembly. Suchcomplex prior art drive assemblies require the stiff central member toset the resonant frequency of the system of counterrotating rotorplatforms since the piezoelectric bimorphs are fixed to the compliantwebs by lossy (i.e. damping) organic adhesive material. Thus, the priorart design is strongly coupled to the case through the stiff centralmembers. In contrast, the rotor and torsion flexure subassemblies arecoupled to the case solely through flexible vanes that possess naturalfrequencies below that of the subassembly, isolating it from externalangular vibrations capable of interfering with resonant frequencyoscillations. Also, the present invention permits a much simpler (bothmechanically and electronically) drive assembly as the common torsionflexure, rather than the stiff central member(s), is the major factor insetting the resonant frequency of the counter-oscillating structure.(Note: In each case, the vanes that support piezoelectric bimorphscontribute 20-30% in establishing the resonant frequency of thecounteroscillating rotor pair.)

A pair of apertures 98, 100 is provided within peripheral mount 96 forreceiving fasteners that fix the mount 96 to a rotor support plat (notshown) in an arrangement such as that illustrated in FIG. 3 above.Referring to the representative rotor arm 82, apertures 102, 104 areprovided for fixing a hybrid substrate (not shown) to the rotor arms foroscillation therewith. Again, the details of such affixation areillustrated in FIG. 3. In this way, the requisite electrical connectionsbetween multisensor electronics and the accelerometers fixed to therotor arms 82, 84, 86 are not jeopardized in operation.

A pair of piezoelectric bimorphs 106, 108 is fixed to opposed sides ofthe vane 94. The bimorphs 106, 108 fixed to two of three vanes per rotorare electrically connected to drive circuitry for generatingappropriately-phased signals to induce simultaneous compression andtension forces at opposite sides of the vane 94, periodicallyreplenishing expended oscillatory energy and causing bowing while thosefixed to opposed sides of the third vane are connected to detectorcircuitry. The dashed line 110 indicates representative bowing of thevane 94 partially in response to the input of signals for inducingtension and compression within the bimorphs 106 and 108 respectively.

Angular rotation of amplitude 8 of the central hub 80 and the affixedrotor arms 82, 84, 86 that carry multisensor accelerometers at theexcited resonant frequency of the rotor pair is maintained by thesimultaneous outward bowing of the vanes of the drive elements 88, 90,92 (indicated by 110, 112 and 114) in a single direction. Suchsimultaneous bowing of the vanes of the three case-fixed mounts producesangular displacement Θ of the hub 80 and attached rotor arms 82, 84, 86in the direction 116. It will be appreciated that the same process takesplace with regard to the rotor fixed to the opposed end of the torsionflexure, with the bowings of the sets of vanes of the rotors 16 and 18always being opposite one another to induce counteroscillation. Thegeneration of signals for producing the requisite paired compression andtension with piezoelectric bimorphs fixed to opposed sides of a vane, aswell as circuitry for receiving the output of the pair of pickoffbimorphs and employing it to regulate the amplitude of oscillation iswell known in the art. Further, the design and poling of paired bimorphsfor generating the desired bowing of associated vanes in conjunctionwith an applied drive signal is also well understood in the art.

As mentioned earlier, it is extremely advantageous that the pairedrotors of a multisensor counteroscillate at a single resonant frequency.In the invention, this is assured by the direct coupling afforded alongthe common axis of rotation through the torsion flexure that is fixed,at its opposed ends, to the hubs of the paired rotors. Such mechanicalcoupling provides a mechanical path for transferring energy directly andreversibly between the counteroscillating rotors. The direct coupling ofenergy through the torsion flexure insures the establishment andmaintenance of a common resonant frequency ω_(D) and, as a consequence,assures that the transfer of energy therebetween is a maximum. Suchenergy readily dampens oscillations of different frequencies whileamplifying those at the common resonant frequency. Thus, in addition toovercoming the substantial complications introduced when a commonresonant frequency of oscillation is not established, the strong forcesestablished for driving the two rotors at a single resonant frequencyassure that oscillations are stabilized at the common resonantfrequency, providing a high Q, and therefore low random walk, system.

While the invention has been described with reference to its presentlypreferred embodiment it is not limited thereto. Rather, this inventionis limited only insofar as defined by the following set of patent claimsand includes within its scope all equivalents thereof.

What is claimed is:
 1. A multisensor comprising, in combination:a) apair of rotors, each of said rotors including a plurality ofradially-directed rotor arms; b) an accelerometer being fixed to each ofsaid rotor arms; c) each of said rotors being arranged to oscillateabout a common axis; d) a drive assembly associated with each of saidrotors for oscillating said rotors substantially 180 degreesout-of-phase with respect to one another; and e) a torsion flexure fordirectly coupling said rotors to one another along said common axis,said torsion flexure having an axis of symmetry coincident with saidcommon axis and comprising (i) a central member and (ii) end members. 2.A multisensor as defined in claim 1 wherein each of said rotors furthercomprises:a) a central hub; b) said rotor arms being joined to andextending radially from said hub; c) a plurality of drive assemblies;and d) each of said drive assemblies comprising a singleradially-directed vane fixed to and extending from said hub and having apair of piezoelectric elements fixed to opposed sides of said vane.
 3. Amultisensor as defined in claim 2 wherein each of said assembliesfurther includes a peripheral mount fixed to the end of said vaneopposite said hub.
 4. A multisensor as defined in claim 3 furtherincluding:a) a case; b) each of said rotors being located within saidcase; and c) said peripheral mounts being fixed to said case.
 5. Amultisensor as defined in claim 4 further including:a) said casecomprising a body having a cylindrical internal cavity for accommodatingsaid rotors; b) a pair of rotor support plats fixed to opposed ends ofsaid cavity; and c) said peripheral mounts being fixed to said plats. 6.A multisensor as defined in claim 5 further including:a) a pair ofhybrid substrates; and b) said substrates being fixed to the rotor armsof said rotors.
 7. A multisensor as defined in claim 2 furtherincluding:a) said central hub has a central aperture; b) said centralaperture is aligned with said common axis; and c) an end member of saidtorsion flexure being fixed to each of said rotors at said aperture. 8.A multisensor as defined in claim 7 wherein said end member is fixed toeach of said rotors by means of an adhesive.
 9. A multisensor as definedin claim 8 wherein said adhesive is a resinous catalytic adhesive.
 10. Amultisensor as defined in claim 7 wherein a laser weld fixes said endmember to each of said rotors.